1. Field of the Invention
The present invention relates to telecommunications, and, in particular, to wireless communications systems conforming to a code-division, multiple-access (CDMA) standard, such as the cdma2000 standard of the IS-95 family of CDMA wireless standards.
2. Description of the Related Art
FIG. 1 shows a block diagram of a conventional CDMA wireless communications system 100. Communications system 100 is assumed to conform to the cdma2000 standard in the IS-95 family of CDMA wireless standards, although the present invention is not necessarily so limited. Communications system 100 comprises an interworking function (IWF) 102 connected to a radio link protocol (RLP) function 104, which is in turn connected to a frame selection/distribution (FSD) function 106, which is in turn connected to one or more base stations 110 via back haul facilities 108 (e.g., T1 lines). Depending on the specific implementation, IWF function 102, RLP function 104, and FSD function 106 may be, but need not be, physically separate functions.
Each base station 110 is capable of simultaneously supporting wireless communications with one or more mobile units 112. FSD function 106 performs a forward-link frame distribution function in which frames of data corresponding to user messages are distributed to the various base stations. In addition, FSD function 106 performs a reverse-link frame selection function in which frames of data received from the various base stations are processed for forwarding on to RLP function 104. In the forward-link direction, RLP function 104 segments user messages received from IWF function 102 into frames of data for distribution by FSD function 106. In the reverse-link direction, RLP function 104 reassembles packets of data received from FSD function 106 into user messages for forwarding on to IWF function 102. IWF function 102 implements a high-level point-to-point protocol (PPP) to perform certain centralized functions for communications system 100 to coordinate and control operations at the various base stations 110. IWF function 102 also functions as the interface between communications system 100 and other communications systems (not shown) to provide a full range of telecommunications services to the mobile units, including voice communications with a remote end unit and/or data communications with a computer server or other nodes of a computer network.
As used in this specification, the term xe2x80x9cmobile unitxe2x80x9d as well as its synonyms xe2x80x9cmobile user,xe2x80x9d xe2x80x9cmobile,xe2x80x9d and xe2x80x9cuser,xe2x80x9d will all be understood to refer to any end node communicating via wireless transmissions with one or more base stations of a wireless communications system, whether that end node is actually mobile or stationary. Also, as used in this specification, the term xe2x80x9cbase stationxe2x80x9d is synonymous with the terms xe2x80x9ccall legxe2x80x9d (or xe2x80x9clegxe2x80x9d for short) and xe2x80x9ccell sitexe2x80x9d (or xe2x80x9ccellxe2x80x9d for short).
The cdma2000 standard supports different modes of data communications. For relatively low rates of data messaging, a fundamental channel (FCH) can handle both signaling and data messaging. Signaling refers to the communications between a mobile and a base station that are used by the mobile and the base station to control the communications links between them, while messaging refers to the information passed through the base station to and from the end nodes of those communications, where the mobile is one of those end nodes. For high-rate data messaging, a supplemental channel (SCH) can be used for data messaging, while the fundamental channel handles the signaling between the mobile and the base station. Alternatively, when an SCH is used for data messaging, the signaling between the mobile and the base station can be handled by a special communications channel called a dedicated control channel (DCCH), which requires less power to transmit than an FCH, which is designed to handle low-rate data messaging in addition to signaling.
FIG. 2 shows a functional block diagram of a portion of communications system 100 of FIG. 1 for a mobile unit 112 operating in soft handoff with three base stations 110. Soft handoff refers to a situation in which a mobile unit is simultaneously communicating with two or more base stations, each of which is referred to as a call leg of those communications. Frame selection/distribution function 106 supports the soft handoff communications between mobile unit 112 and the three base stations 110.
During normal voice communications, mobile 112 transmits voice messages using a reverse-link fundamental channel. Each of the three base stations 110 in soft handoff with mobile 112 receives the reverse-link FCH, accumulates voice messages into reverse-link packets, and transmits the reverse-link packets over back haul 108 to FSD function 106. FSD function 106 receives the reverse-link packets from all three base stations, identifies sets of corresponding reverse-link packets (one reverse-link packet from each base station corresponding to the same voice messages received from the mobile), and selects one reverse-link packet from each set of corresponding reverse-link packets to transmit to the rest of the wireless system for eventual transmission to the remote end of the call (e.g., a connection with a regular PSTN user or possibly another mobile unit in communications system 100).
At the same time, FSD function 106 receives forward-link packets containing voice messages from the remote end of the call intended for mobile unit 112. FSD function 106 distributes copies of each forward-link packet to all of the base stations currently in soft handoff with the mobile. Each base station transmits the forward-link packets to mobile unit 112 using a different forward-link fundamental channel. Mobile unit 112 receives all three forward-link FCHs and combines corresponding voice messages from all three forward-link FCHs to generate the audio for the person using mobile unit 112.
The timing of the distribution of the copies of the forward-link packets from FSD function 106 to the three base stations is critical, because mobile unit 112 needs to receive each set of corresponding voice messages from all three forward-link signals within a relatively short period of time in order to be able to combine all of the corresponding voice messages together. Similarly, FSD function 106 needs to receive all of the corresponding reverse-link packets from the different base stations within a relatively short period of time in order to coordinate the selection of packets for further processing. In order to satisfy these forward-link and reverse-link timing requirements, whenever a new call leg is added at a base station (i.e., whenever a new base station begins communications with a particular mobile unit in soft handoff), special synchronization procedures are performed between the base station and FSD function 106, e.g., in order to ensure proper synchronization of that base station""s forward-link transmissions with the forward-link transmissions from the other base stations currently participating in soft handoff with the mobile. These synchronization procedures involve specific communications back and forth between the base station and the FSD function over the back haul.
Although a fundamental channel can support some modest amount of data messaging in addition to voice messaging, the cdma2000 standard also supports high-speed data messaging via supplemental channels. According to the cdma2000 standard, since data messaging is typically bursty (i.e., intermittent), as opposed to the continuousness of voice messaging, supplemental channels are established and maintained only for the duration of each data burst. During a burst of data messaging via an assigned SCH, the mobile unit is said to be in an active state. Between bursts of data messaging when no SCH is currently assigned, but when an FCH (or DCCH) is assigned, the mobile unit is said to be in the control hold state.
Analogous to the use of a fundamental channel for voice and/or low-speed data messaging, high-speed reverse-link data messages are transmitted by mobile unit 112 using a reverse-link supplemental channel. Each base station currently operating in soft handoff with the mobile unit receives the reverse-link SCH and generates reverse-link packets of data messages for transmission to FSD function 106 via the back haul. FSD function 106 receives the reverse-link packets from all of the base stations and selects appropriate reverse-link packets for transmission to the remote end of the call (which, in the case of data messaging, may be a computer server).
Similarly, FSD function 106 receives forward-link packets of data messages intended for mobile unit 112 and coordinates the distribution of those forward-link packets via the back haul to the appropriate base stations for coordinated transmission to the mobile via assigned forward-link supplemental channels. In addition to the synchronization processing between each base station and FSD function 106 required to meet the timing requirements for receiving messages at the mobile, in data communications, the base stations need to coordinate their operations to ensure that they all transmit their forward-link SCHs to the mobile at the same data rate. This requires the base stations to communicate with one another via the back haul whenever a new burst of forward-link data is to be transmitted to the mobile unit requiring new SCHs to be assigned.
The time that it takes to change the status of a mobile unit from the control hold state to the active state is called the reactivation time. In prior-art IS-95 CDMA systems, the reactivation time includes the time required to assign a new channel to the mobile and the time required to synchronize each base station with the frame selection/distribution function. When the new channel is a supplemental channel to be used for data transmission to a mobile unit in soft handoff, the reactivation time also includes the time required for the different base stations to coordinate their forward-link transmission data rates. In general, the longer the reactivation time, the lower the data throughput of the wireless system. As such, it is desired to keep reactivation time as low as practicable.
The present invention is directed to a back haul architecture that effectively reduces the reactivation times for forward-link data transmissions over CDMA wireless communications systems. In particular, according to the present invention, forward-link data messages can be transmitted to a mobile unit using a forward-link supplemental channel operating in simplex mode (i.e., forward-link transmissions from a single base station) independent of whether or not the mobile unit is simultaneously operating in soft handoff to receive other forward-link transmissions via fundamental channels (or dedicated control channels) and independent of whether multiple base stations are simultaneously operating in soft handoff to receive any reverse-link communications from the mobile via a fundamental channel, a dedicated control channel, and/or a supplemental channel.
According to the present invention, in addition to a conventional circuit-oriented frame selection/distribution (FSD) function, a packet-oriented FSD function is implemented to handle the data messages being transmitted to and from the mobile unit. The packet-oriented FSD function transmits forward-link data messages directly to a primary base station, without first coordinating with the primary base station regarding time, duration, or rate of transmission over the air interface. The primary base station then decides whether to transmit the forward user data to the mobile unit using a supplemental channel or a fundamental channel.
If there is not a lot of forward user data to be transmitted, the primary base station can decide to transmit the forward-link data messages to the mobile unit using a fundamental channel. In that case, when the mobile is in soft handoff, the forward-link data needs to be provided to the one or more other (secondary) base stations, so that all of the call legs can transmit the data to the mobile unit at the same time. The present invention provides back haul reverse and forward packet formats to support the transmission of forward user data from the primary base station to the secondary base stations via the circuit-oriented FSD function.
Otherwise, the primary base station can decide to transmit the forward-link data messages to the mobile unit using a supplemental channel. In that case, whether or not the mobile unit is otherwise operating in soft handoff (e.g., via a fundamental channel), the primary base station assigns a supplemental channel and transmits the forward user data to the mobile unit using that supplemental channel in simplex mode. Because the forward-link supplemental channel is operated only in simplex mode, there is no need to coordinate either the timing or the data rate of forward-link data transmissions with any other base stations, and no such coordination needs to be performed in order to receive the data from the FSD function. As such, the reactivation time needed to resume active forward-link data transmissions using a supplemental channel from the control hold state is less than the corresponding reactivation time under the prior art, which does require coordination of timing and data rate between all base stations in soft handoff.
In one embodiment, the present invention is a wireless communications method, comprising the steps of (a) receiving forward-link data at a frame selection/distribution (FSD) function of a wireless communications system; (b) transmitting the forward-link data from the FSD function to a primary base station of the wireless communications system without first coordinating between the FSD function and the primary base station regarding transmission of the forward-link data from the primary base station over an air interface; and (c) determining at the primary base station whether to transmit the forward-link data over the air interface using a fundamental channel or a supplemental channel.
In another embodiment, the present invention is a wireless communications system, comprising an FSD function configured to (a) receive forward-link data; and (b) transmit the forward-link data to a primary base station of the wireless communications system without first coordinating between the FSD function and the primary base station regarding transmission of the forward-link data from the primary base station over an air interface.
In another embodiment, the present invention is a wireless communications system, comprising a primary base station configured to (a) receive forward-link data; and (b) determine whether to transmit the forward-link data over an air interface using a fundamental channel or a supplemental channel.